Renal – Kidneys Excretion and Osmoregulation Chapter 18.6 page 513

Excretion and Osmoregulation In mammals the removal of metabolic waste materials (excretion) and much of the regulation of body fluids (osmoregulation) are achieved by the excretory system (Renal – Kidneys) The separation of excretory substances from blood and the conservation of water are performed in the kidneys.

The Kidneys

The concentrations of chemicals dissolved and suspended in the cytoplasm, intercellular fluid and circulating fluid must be maintained within a particular range. This is achieved by osmoregulation and the excretion of metabolic wastes. Most animals use an excretory system to maintain the homeostasis.

Much metabolic waste material is removed from the animal through surfaces in contact with the environment. Carbon dioxide diffuses from the blood to the alveoli of the lungs and through skin surfaces. Salts may be removed through the skin; for example, by sweat in mammals. Salts are also secreted into the large intestine. The kidneys have a significant role in the regulation of salt levels in the body, along with fluid levels.

Removal of Ammonia Amino acids derived from cell replacements and those taken in as surplus to the body’s requirements cannot be stored, and are broken down by the liver in vertebrates. In this process the nitrogen containing amino group is removed. The products either enter the respiratory pathway or are converted to a storage material and ammonia. Ammonia is highly toxic to animal cells and must either be rapidly removed from the body or converted to a less toxic compound such as urea or uric acid.

Removal of Ammonia Since mammals are primarily terrestrial animals, mammals must conserve water and thus their nitrogenous waste material must be in a relatively soluble, non-toxic form – they excrete urea. The separation of excretory substances from blood and the conservation of water are performed in a pair of kidneys

Formation of Urine Waste materials dissolved in water (urine) passes from the kidneys through the ureters to be stored in the bladder. The urine eventually passes to the external environment through the urethra.

STRUCTURE AND FUNCTION OF THE HUMAN KIDNEY Chapter 18.6 page 514

The excretory system consists of a pair of kidneys which remove excretory products from the blood plasma. The urine so formed passes through the ureters to the bladder for storage. When the bladder is distended with urine, it releases the urine through a sphincter of striated muscle to a median urethra which discharges to the exterior.

Structure of the kidney The two kidneys are found at either side of the mid-dorsal line of the abdominal cavity. The kidney is a bean-shaped organ. It is enclosed by a capsule of connective tissue and composed of two layers of tissue: an outer, darker-coloured cortex and an inner, lighter-coloured medulla, surrounding a central cavity.

Structure of the kidney The Kidneys are connected to the circulatory system via the renal arteries and veins and to the bladder via the ureters. Within the central cavity the top of the ureter expands to form the pelvis. The medulla is divided into lobes of approximately conical shape which are called the pyramids.

Structure of the Nephron Nephrons are functional units of the kidney. There are between 1-2 million nephrons per kidney. Each nephron consists of an elongated tubule closely associated at one end with a group of blood capillaries (the glomerulus) via a cup-shaped Bowman’s capsule and opening at the other end into a collecting duct. Many nephrons open into each collecting duct, which drains into the pelvis and thus out through the ureter.

The nephron tubule has distinct portions The nephron tubule has distinct portions. The Bowman’s capsule, wrapping around the glomerulus, opens into the proximal convoluted tubule which is highly coiled. This leads to the U-shaped loop of Henle and on to the coiled distal convoluted tubule, which opens into the collecting duct. Only the loop of Henle and the collecting duct are in the medulla. The rest of the nephron is contained within the cortex of the kidney.

Proximal Convoluted Tubule The proximal tubule as a part of the nephron can be divided into an initial convoluted portion and a following straight (descending) portion. Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peri-tubular capillaries, including approximately two-thirds of the filtered salt and water and all filtered Organic solutes (primarily Glucose and Amino acids).

The Loop of Henle The Loop of Henle, also called the nephron loop or the loop of Hundley, is a U-shaped tube that extends from the proximal tubule. It consists of a descending limb and ascending limb. It begins in the cortex, receiving filtrate from the proximal convoluted tubule, extends into the medulla as the descending limb, and then returns to the cortex as the ascending limb to empty into the distal convoluted tubule. The primary role of the loop of Henle is to concentrate the salt in the tissue surrounding the loop.

The Descending Limb - Loop of Henle Considerable differences distinguish the descending and ascending limbs of the loop of Henle. The Descending Limb is permeable to water and noticeably less impermeable to salt, and thus only indirectly contributes to the concentration of the interstitium. As the filtrate descends deeper into the hypertonic interstitium of the renal medulla, water flows freely out of the descending limb by osmosis until the tonicity of the filtrate and interstitium equilibrate. Longer descending limbs allow more time for water to flow out of the filtrate, so longer limbs make the filtrate more hypertonic than shorter limbs.

The Ascending Limb - Loop of Henle Unlike the descending limb, the thin ascending limb of loop of Henle is impermeable to water, a critical feature of the countercurrent exchange mechanism employed by the loop. The ascending limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives countercurrent exchange. In passing through the ascending limb, the filtrate grows hypotonic since it has lost much of its sodium content. This hypotonic filtrate is passed to the distal convoluted tubulein the renal cortex.

The Distal convoluted tubule The distal convoluted tubule has a different structure and function to that of the proximal convoluted tubule. Cells lining the tubule have numerous mitochondria to produce enough energy (ATP) for active transport to take place. Much of the ion transport taking place in the distal convoluted tubule is regulated by the endocrine system. Significant concentration of urine can occur in the DCT.